1. Field of the Invention
The present invention relates to a piezoelectric thin film element and a piezoelectric thin film device, using a piezoelectric thin film of alkali-niobium oxide series.
2. Description of the Related Art
A piezoelectric ceramics is processed into various piezoelectric elements in accordance with various purposes, and is widely used particularly as a functional electronic component such as an actuator for generating deformation by adding a voltage to the piezoelectric elements, and a sensor for detecting the voltage generated by deformation of the piezoelectric elements. As the piezoelectric ceramics used for the purpose of use of the actuator and the sensor, a dielectric material, being a lead-based material, having excellent piezoelectric characteristics, and particularly a PZT-based perovskite-type ferroelectric ceramics expressed by a composition formula: Pb(Zr1-xTix-1)O3 is widely known heretofore. PZT-based piezoelectric ceramics is ordinarily formed by sintering an oxide of each of the constituents. With a progress of miniaturization and high performance of each kind of electronic component, the miniaturization and high performance of the piezoelectric elements are also strongly requested.
However, in a piezoelectric material fabricated by a sintering method, being a conventional method, as a thickness thereof becomes smaller, particularly as the thickness becomes nearly about 10 μm, a thickness of the piezoelectric thin film corresponds to almost a size of each crystal grain constituting the thin film, and an influence on the piezoelectric characteristics, etc, can not be ignored. Therefore, there is a problem that variation or deterioration of characteristics is remarkable. Therefore, in order to prevent such a problem, a forming method of the piezoelectric ceramics using a thin film technique instead of the sintering method, has been studied in recent years. Recently, a PZT thin film formed on a silicon substrate by sputtering has been put to practical use as a piezoelectric thin film of an actuator for an ink jet printer head with high-speed and high-definition.
Meanwhile, a piezoelectric sintered ceramics or a piezoelectric thin film made of PZT contains lead by about 60 to 70 wt %, and therefore is not preferable in terms of an ecological viewpoint and pollution control. Therefore, in consideration of an environment, development of the piezoelectric ceramics not containing lead is desired. At present, various non-lead piezoelectric materials are studied, including potassium sodium niobate (also described as “KNN” hereafter) expressed by a composition formula (K1-xNax)NbO3(0<x<1), composed of K (potassium), Na (sodium), Nb (niobium), and O (oxygen) (for example, see patent document 1 and patent document 2). The KNN is a material having a perovskite structure and shows relatively excellent piezoelectric characteristics for a non-lead (lead-free) material, and therefore is expected as a strong candidate of a non-lead (lead-free) piezoelectric material.
(Patent Document 1)
    Japanese Patent Laid Open Publication No. 2007-184513(Patent Document 2)    Japanese Patent Laid Open Publication No. 2008-159807
A KNN thin film is formed on a silicon substrate by a film formation method such as sputtering, Sol-Gel method, and aerosol deposition, and there is a report that a piezoelectric constant d31=−100 pm/V, being characteristics of a practical level, can be obtained. However, a method for realizing the KNN thin film having excellent piezoelectric characteristics stably has not been established, and at present, application to a practical use is difficult. In order to widely apply the KNN thin film to an ink jet printer head, etc, at least the piezoelectric constant d31 of −90 pm/V or more, preferably −100 pm/V or more needs to be stably realized. (In this specification, in a case of the piezoelectric constant d31, being a negative value, −90 [pm/V] or more of the piezoelectric constant d31 means that the piezoelectric constant d31 takes values such as −90, −100, −110 [pm/V], etc, namely, an absolute value |d31| is 90 [pm/V] or more.)